The present invention relates to a method for producing extensible paper, a plant for implementing the method, a product obtained by the method, and a paper material obtained from the product.
Extensible paper is a known paper which, because of special treatment during its production, presents considerable extensibility both in the longitudinal direction (i.e. in the direction of its advancement along the production line) and in the transverse direction (i.e. in the direction perpendicular to the preceding). This treatment consists essentially of passing the paper web, not yet formed and presenting a moisture content of about 35%/45%, between two rollers rotating at different speeds. One of these rollers, generally the lower roller, is made of rubber and is rotated at lower speed, while the upper roller is made of steel and comprises in its cylindrical surface a continuous spiral-shaped groove. The different material nature and the different speed of the two rollers results in a sort of longitudinal accumulation of the paper forming material and prepares it for longitudinal extensibility, by an amount which can reach 15-20%. At the same time, the spiral groove performs a double function: on the one hand it causes a sort of transverse accumulation of the material forming the paper, to prepare it for transverse extensibility, by an amount which can reach 10-15%. On the other hand the spiral groove contributes to maintaining longitudinal advancement of the processed paper web along the machine.
The longitudinal and transverse paper accumulation as a result of its passage between the two rollers, and the particular method by which this is achieved, results in the formation, in the paper web leaving the machine, of a series of folds, compactions or heaps which extend for various lengths with a pattern reflecting the surface of the steel roller, and cover the entire surface of the web. In their turn, these folds define in the paper a sort of accentuated macroscopic roughness, giving rise to serious problems which substantially limit its use.
One of these problems, related to the widespread use of this type of paper in the packaging field, in which it has to be coupled to plastic film or other impermeabilizing layers, is that the accentuated roughness makes it more difficult for the impermeabilizing layer to correctly and uniformly adhere to the extensible paper web, resulting in possible detachment between the two when the combination is subjected to deformation to produce three-dimensional packs.
Another problem is that because of the particular undulated geometry of the product web, the accentuated roughness results in a substantial decrease in rigidity, i.e. in bending resistance, which is very important for packaging materials.
Another problem is that although the accentuated roughness is far removed from the crinkling used in the past to produce extensible paper, it still forms an obstacle to the deposition of ink, and essentially an obstacle to correct printing, which on the contrary would require the smoothest possible surface.
The invention proposes to eliminate these problems, i.e. to provide a paper which simultaneously presents considerable extensibility characteristics while being substantially free of surface roughness, measurable in terms of order of magnitude on the Bendsen scale, consequently making it uniformly rigid, easily coupled to impermeabilizing layers, and printable without particular problems and with satisfactory results on traditional printing machines.